feat(pytest): support extra command-line arguments

[March-77]

Summary

• add an args field to the run-code API, internal runner model, and Python client
• pass pytest arguments with shell-safe quoting so each caller-provided token keeps its argv boundary
• install pytest-json-report in the Python runtime so the issue's JSON-report example works
• cover API propagation, shell metacharacter quoting, and fetching a generated JSON report

Testing

• targeted API/runner tests: 2 passed
• reproduced the issue's pytest JSON-report command directly: 1 passed; generated report parsed successfully
• changed-file isort, YAPF, pycln, compileall, and pip check: passed
• full make check was not run locally because the sandbox test harness requires Linux resource, /bin/bash, and the prebuilt sandbox-runtime environment; the added end-to-end test exercises that environment in project CI

Fixes #78

[March-77]

Revalidated on 2026-08-23 against main 469de08:

• The PR remains clean and mergeable; no reviews or CI checks are currently present.
• Main still does not expose extra pytest arguments, and the RunCodeRequest to CodeRunArgs to run_pytest interface is unchanged.
• Targeted Python 3.11 validation passed for API propagation, shell-safe argv preservation, and an actual pytest run generating and fetching the JSON report.
• PRs feat(pytest): support extra command-line arguments #81 and fix(runner): prevent subprocess pipe deadlocks #82 are independent: they share the same base and modify disjoint files.

No code-only refresh commit was added. Maintainer review would be appreciated.